Field of the Invention
The present invention relates to a stop apparatus used in a lens apparatus for image taking, in particular to a stop apparatus having an aperture diameter varied by rotational operation. The present invention also relates to a lens apparatus and an image pickup apparatus having such a stop apparatus.
Description of the Related Art
There has been known a stop apparatus including a plurality of stop blades each having a pivot axis arranged concyclically at regular angular intervals and having a stop aperture defined by edges of the overlapping stop blades, which is varied by rotations of the stop blades about their respective pivot axes. This type of stop apparatus includes a pivot pin and a drive pin provided for each of the stop blades, and the position of the drive pin relative to the pivot pin planted at a fixed position is guided by a cam groove provided in a disk-like cam plate.
For example, Japanese Patent Application Laid-Open No. 2004-294678 discloses a stop apparatus having six stop blades. In this apparatus, pivot pins for the six stop blades are fitted to engage holes provided on a stop support member, and the stop aperture is varied by rotating a cam plate having six cam grooves engaging with the drive pins of the respective stop blades. When this stop apparatus is built in a lens apparatus, the rotary operating member and the cam plate are linked to allow manual control of the light quantity.
The shape of the stop aperture affects the quality of bokeh (the aesthetic quality of blur) around the main subject of picked up images, and the closer to a circle the shape of the stop aperture is, the more excellent the bokeh is rendered. A widely known method of making the stop aperture shape close to a circle is to increase the number of the stop blades. Japanese Patent Application Laid-Open No. 2012-123299 discloses a stop apparatus having an aperture shape close to a circle formed by fourteen stop blades.
For the above reason, the stop apparatus is required to have a stop aperture shape close to a circle. Moreover, the stop apparatus is required to have a large rotational operation angle in order to allow fine adjustment of the light quantity by manual operation. In particular, when applied to a lens apparatus such as a cinema lens that is required to form beautiful images and to allow fine light quantity adjustment, the stop apparatus is required to meet both the requirements.
A widely known method of enlarging the rotational operation angle is to provide a differential mechanism including a roller operating between a rotary member in the lens apparatus and the manual operation member to enlarge the rotation angle of the manual operation member relative to the rotation angle of the rotary member in the lens apparatus, as disclosed in Japanese Patent Application Laid-Open No H03-182709.
However, it is difficult in the prior art apparatuses disclosed in Japanese Patent Application Laid-Open Nos. 2004-294678 and 2012-123299 mentioned above to achieve a large rotational operation angle while using a large number of stop blades because of a restrictive condition that interference of cam grooves that guide the drive pins of the stop blades with each other must be prevented. Therefore, conventional stop apparatuses that use a large number of blades in order to achieve a stop aperture shape close to a circle do not allow fine adjustment of the light quantity using an operation member, causing inconvenience in image shooting.
Applying the structure disclosed in Japanese Patent Application Laid-Open No. H03-182709 mentioned above leads to a problem of increase in the overall size of the stop apparatus including the differential mechanism.